This invention relates to an electronic musical instrument capable of synthesizing a musical tone in accordance with a fixed formant by frequency modulation.
Natural musical instruments are known to have their own fixed formants peculiar to structures of the musical instruments such as a configuration of a sound-board in the case of a piano. A fixed formant exists in a human voice also and this fixed formant characterizes a tone color peculiar to a human voice. In order to simulate a tone color of a natural musical instrument or a human voice in an electronic musical instrument, a musical tone must be synthesized in accordance with a fixed formant peculiar to the tone color.
There have been proposed several methods of realizing a fixed formant in an electronic musical instrument. In these methods, one employing a frequency modulation computation is advantageous over other methods in respect of cost saving and simplification of structure. Prior arts relating to the present invention are listed up as follows:
a. U.S. Pat. No. 4,018,121 entitled "Method of Synthesizing a Musical Sound" PA1 b. Japanese Patent Preliminary Publication No. 1980-18623 entitled "Electronic Musical Instrument"
The U.S. Pat. No. 4,018,121 discloses synthesizing of a tone of a desired spectrum structure by performing frequency modulation computation in an audio frequency range. The Japanese Patent Preliminary Publication No. 1980-18623 discloses synthesizing of a tone in accordance with an almost complete fixed formant by utilizing such frequency modulation computation.
In the electronic musical instrument of Japanese Patent Preliminary Publication No. 1980-18623, a formant is synthesized by conducting frequency modulation and using a frequency which is an integer multiple of a fundamental frequency designated by depression of a key as a carrier and the fundamental frequency as a modulating wave. In view of the fact that a center frequency of a fixed formant is not necessarily an integer multiple of a fundamental frequency of a depressed key, a harmonic frequency which is nearest to the formant center frequency is calculated and a formant having the calculated harmonic frequency as a central component is synthesized by frequency modulation, using the calculated harmonic frequency as a carrier. This necessitates a rather complicated computation circuit for determining a frequency which is nearest to the formant center frequency from among harmonic frequencies of the tone designated by the depression of the key (in other words, a converter for converting the formant center frequency to the nearest harmonic frequency). Conversion of the fixed formant center frequency to the nearest harmonic frequency is required for adjusting frequencies of the carrier wave and the modulating wave used in the frequency modulation computation to harmonic frequencies of a desired tone so that sidebands obtained by the frequency modulation will constitute harmonic component of this tone.
In the electronic musical instrument disclosed in the Japanese Patent Preliminary Publication No. 1980-19623, a formant which is more or less shifted from a desired fixed formant is synthesized if a center frequency of an original formant does not coincide with the nearest harmonic frequency. The amount of this shift poses a problem in a case where the fundamental frequency (f.sub.0) of the depressed key is relatively high. An example of a spectrum envelope appearing in a case where the fundamental frequency (f.sub.0) is low is shown in FIG. 1(a) whereas an example of a spectrum envelope appearing in a case where the fundamental frequency (f.sub.0) is high is shown in FIG. 1(b). In each of these figures, a spectrum envelope of an object fixed formant to be synthesized is indicated by a solid line and a spectrum envelope of a formant which is actually synthesized by the prior art is indicated by a broken line. If the fundamental frequency (f.sub.0) is low, interval of harmonic frequencies (f.sub.0, 2f.sub.0, 3f.sub.0. . . ) is relatively narrow and, accordingly, differences between center frequencies (f.sub.f1, f.sub.f2) of desired fixed formants and harmonic frequencies (3f.sub.0, 8f.sub.0) in the vicinity of the center frequencies (f.sub.f1, f.sub.f2) are not so large, as shown in FIG. 1(a), and differences between formants synthesized about the harmonic frequencies (3f.sub.0, 8f.sub.0) and the desired fixed formants are of an insignificant amount. If, however, the fundamental frequency (f.sub.0) is high, the interval of the harmonic frequencies (f.sub.0, 2f.sub.0, 3f.sub.0 . . . ) is so wide that, as shown in FIG. 1(b), differences between the center frequencies (f.sub.f1, f.sub.f2) of desired fixed formants and the harmonic frequencies (f.sub.0, 2f.sub.0) in the vicinity thereof are widened and formants synthesized about the harmonic frequencies (f.sub.0, 2f.sub.0) are shifted from the desired fixed formants to a large extent resulting in deterioration in tone quality of a produced tone. For example, an original level of the harmonic frequency 2f.sub.0 shown in FIG. 1(b) is l.sub.0 but the level of the frequency becomes L which is much higher than l.sub.0 due to shifting of the formant as indicated by the broken line with a result that the desired tone color cannot be obtained. For eliminating this disadvantage in the prior art, a level correction circuit must be additionally provided to correct the error in the signal level produced by the shifting of formant. This level correction circuit has to be of complicated construction for detecting frequency difference between the formant center frequency and the nearest harmonic frequency and applying a suitable level correction in accordance with the detected frequency difference.